Radial lip oil seals having an elastomeric body portion thermally bonded to an annular, usually cup-shaped, metal case have been in use for many years. Such seals are generally used between relatively rotatable parts to seal oil or grease in a predetermined location for lubrication. Common applications for these seals include sealing vehicular engine crankshafts and transmission shafts. In each case, the elastomeric body portion of the seal is usually designed to include an annular flex portion of reduced cross-sectional thickness bonded to the metal case member and located intermediate the seal lip and metal case. The purpose of this flex section is to allow the seal lip to stay in continuous, intimate contact with the shaft it is to seal despite any lack of concentricity between the relatively rotating members, e.g. the rotating shaft and the stationery engine block into which the annular metal case member is usually press-fit or otherwise nonrotatably secured.
It is also known that for certain applications, particularly off-road vehicular applications such as earth hauler and tractor applications, it is desirable that the elastomeric oil seal include at least one secondary seal lip, axially spaced from the primary seal lip, for the purpose of excluding dirt, dust and other particulate type contaminants from the primary seal lip. During the molding process of such "dual lip" seals, the anchoring portion of the annular metal case is clasped between an upper and lower die and extends within a molding cavity formed by the two. A common annular ring of elastomeric prep material is heated within the mold and subsequently flows within the die cavity filling it, forming the primary and secondary seal lips and a common flex section opposite the entrapped anchoring portion of the metal case. Upon cooling and stripping the seal from the mold, the two lips are seen to be joined to the metal case by the common flex section. Further, that portion of the elastomeric body portion intermediate the primary and secondary seal lips also functions as a secondary flex section.
As a result of the common flex section, each seal lip in operation is relatively independent of the case member. That is, each seal lip can accommodate shaft eccentricity with the axis of the metal case member and with the bore in which it is located. Further, as a result of the second or intermediate flex section, the primary and secondary sealing lips are fairly operationally independent of one another. That is, because the elastomeric material is relatively soft and pliable, each seal lip can accommodate shaft surface irregularities appearing only at that one seal lip, such as some localized out of roundness, without affecting the performance of the other seal lip. For certain seal designs, the common flex section and the secondary flex section may be one and the same.
U.S. Pat. No. 2,992,027 presents a discussion of this same operational characteristic and suggests that the design of the flex section itself, which is intermediate the primary and secondary seal lips, may be altered to assure independent action.
Most recently, the wear characteristics of this general seal design have been enhanced by lining the sealing surface of each seal lip with a low friction, high wear resistant material such as polytetrafluoroethylene (PTFE) or the like. An example of this design is shown in U.S. Pat. No. 4,171,561. While such a design enhances at least the long life characteristics of the oil seal, it has been found that the benefit of the relatively independent action of the primary and secondary seal lips relative to one another has been diminished by the stiffness of the PTFE liner which extends across the secondary flex section.
Consequently, attempts have been made to economically provide an oil seal of this type with both seal lips lined with low-friction material but without sacrificing the independent flexibility of each.
One such design includes a method of manufacture which allows the use of a single annular prep material piece and a single PTFE liner wafer with the inherent cost and molding process advantages of such a design, namely that shown in U.S. patent application Ser. No. 347,920, assigned to the assignee of the present invention. By such technique, the PTFE wafer is split during mold closure into two separate annular rings, one for the primary lip and the other for the secondary lip. Unfortunately, the reliability of the freed wafer being split and locating consistently in the final design position by the hydrodynamic force of the flowing elastomeric prep material can not always be assured.